

rilKMlsTKY. 



riNO 



glf*immt 09. Into another of the test-tube* poor a 

 few drops of tincture of gall*, which U nold by all chemists 

 and druggist* ; a bUck colour will 1. ntlordi-d. 



luto another teat-tulx) drop some 

 solution <>f carbonate of soda ; a dark-green precipitate 

 will fall down, which, on being exposed to the air, will 

 turn to a red colour. 



It follows, from the above experiment*, that tho pre- 

 wnoa of iron may be d .r--<- methods'; each 



tact employed affording a different result NVe have 

 chosen irt.u as the subject of our illustration, because 

 the characteristics of its presence are so easily noticed. 



But such is not the case with many other gubst ices. 



instance, white powders or precipitates are thrown 



down from many solutions ; and to awtingniah between 



tlii-ui U by no means easy. Each precipitate, however, 



has a distinctive character, as the following experiments 



rare. 



Experiment 71. Add together, in a test-tube, soluti. -us 

 of common salt and nitrate of silver. It will be noticed 

 that the white precipitate has a peculiar appearance, 

 like curdled milk. 



-Add a little solution of chloride of 

 barium to one of sulphate of soda. A white precipitate 

 will be produced in a pulverulent form. 



Now these two precipitates, although both white, are 

 readily distinguished by their physical :\\ ] >earance ; and 

 may be still more so by the action of li-,'ht ; for if that. 

 of the silver be exposed to the rays of the sun, it will 

 turn black, whilst the precipitate of sulphate of baryta, 

 in the last experiment, will maintain its white appearance. 



In precipitates which are coloured, very little difficulty 

 exists, as the colours vary so considerably. But in some 

 cases great caution U required in the amount of test 

 which is used ; because an excess will not onlv re-dissolve 

 a precipitate, but will even prevent its production. Of 

 tliis the following are two instances : 



,in-i,l 73. Add a little soda-water to some lime- 

 water. Carbonate of lime will be thrown down. After- 

 wards add soda- water in abundance, when the exctts 

 of carbonic acid will at once re-dUsolve the precipitate.* 



Experiment 74. To a solution of corrosive sublimate, 

 add a few drops of one of iodide of potassium. A yellow 

 precipitate will be afforded. Ad'l still more of the iodide, 

 when the biniodide of mercury, which has a red colour, 

 will In produced. Afterwards add the iodide of potas- 

 sium in excess, and the precipitate will be entirely dis- 

 solved. A few drops of the corrosive sublimate being 

 added to the liquid, will first produce a red, and then a 

 yellow iodide, which would be again dissolved by an 

 excess of the iodide of ]K>tassium. 



Thesf nts will im; ress on the mind of the 



student the necessity of caution and care in testing ; 

 and will, we trust, correct the almost invariable practice 

 of tin- tyro in using In rye quantities of any test. 



We may now show how accidental circumstances will 

 often cause considerable difficulty to the careless or 

 ignorant person ; and even experienced chemists may, 

 through the neglect of proper precautions, be led into 

 error. Not only is it required that proper tests be em- 

 ployed, but that they should also be perfectly pure, and 

 tree from all foreign matter. 



'I 75. Dissolve a little nitrate of silver in 

 distilled water, so as to obtain a perfectly transparent 

 solution. Kill a test-tul>e with distilled, and another 

 with river-water ; and to each add a few drops of tho 

 solution of the silver salt. The distilled water will all'. .1 ,1 

 no change, but the river- water .villgivo an opal-coloured 

 mg to the presence of common salt and 

 other chlorides. 



.iv hero remark on the necessity of great cleanli- 

 ness in all vowels used for testing purposes. In long 

 laboratory experience, we have noticed numerous failures 

 occurring, to young analysts through neglect in this re- 

 spect In very accurate experiments, test-glasses, <tc., 

 should invariably be washed by distilled water, lest some 

 of the salts contained in common water should remain 

 on the surface after washing. 



Sw , p. 110. 



Occasionally precipitates do not readily exhibit ' 

 selves, as may be seen in the following i-\- intent : 



.'i. Adda little tartaricacid toa solution 

 of carbonate of potass, in a test-tube perfect iv free from 

 scratches, Arc., on the glass surface. No pn -ei]iilate will 

 be noticed. Now nib the surface of tho glass by m 

 of a stirrer, or rough substance of any kind. A ] 

 pitate, composed of minute crystals, will soon be pro 

 duco<l, owing to an angular surface, which is favourable 

 to crystallisation b in.; carried on. 



Occasionally chemie il changes soon take place in sub- 

 stances kept as tests. An instance of this may be found 

 in the solution of iron ; for if the proto-sulphate, in a 

 pure state, be tested by means of prussiate of potass, a 

 light greyish-blue precipitate will be produced. If, how- 

 ever, tho solution of tho iron-salt has been kept for any 

 length of time in a bottle half filled with it, the 

 oxygen of the air in tho bottle will have converted the 

 protoxide into the peroxide ; and tho precipitate, by 

 means of the prussiate of potash, will have a dark-blue 

 colour. 



Many metallic oxides, ic. , maybe readily distinguished 

 by the colour which they afford to llama ; of which tho 

 following are instances : 



Experiment 77. Dissolve common salt, in spirits of 

 wine, and ignite the solution ; a yellow flame will be 

 atl'o nled. 



Experiment 78. Dissolve in the same way, separately, 

 the nitrates of baryta and strontia. These will all'oid, 

 respectively, a green and red flame. 



In using the blow-pipe, these indications are of ex- 

 treme importance, and .ire relied on by the chemist when 

 minute quantities of bodies alone can bo obtained. 



Hydro-sulphuric acid, and hydro-sulphate of ammonia, 

 are much employed as tests for metals. Either may be 

 produced by the methods described at a previous page ;t 

 and the study of their indications will be of the utmost 

 importance to the student, and absolutely necessary 

 before he commences a course of analytical chemistry. 



The solution of many metals may be decomposed, and 

 the metal set free by immersing slips of other metals ; of 

 which the following afford instances : 



,'>. Immerse a piece of silver foil in a 

 solution of chloride of platina. The latter metal will be 

 deposited on the silver as a black powder. 



Experiment 80. Into a solution of nitrate of silver, 

 introduce a piece of polished copper, when the silver will 

 be at once reduced to a metallic state. 



Experiment 81. Dip a piece of clean iron into a solu- 

 tion of sulphate of copper. The latter metal will be at 

 once precipitated on the iron as a brown powder, which 

 consists of pure copper. 



The foregoing experiments will familiarise the minds 

 of our readers with the general principles which must be 

 followed in examining the characteristics of metals, ko. , 

 by testing. For the methods of bringing them into 

 solution, our previous observations and instructions, 

 under their separate heads, must bo followed ; and we 

 strongly advise all who are desirous of acquiring and 

 maintaining tho knowledge of the properties of metals, 

 to keep the precipitates produced in the experiments by 

 them in a dry state. Such should be put into corked 

 test-tubes, and properly Libelled, with the name in full, 

 together with the symbol showing their constitution. 

 By such an arrangement, the results of previous investi- 

 gations are kept ready for reference ; and in the course 

 of further investigations, many difficulties may thereby 

 be avoided. In our earlier days, in chemical study, we 

 adopted this plan ; and in subsequent investigations of 

 a more scientific nature, have often arrived at speedy 

 conclusions ; whilst our companions had to refer to lialf- 

 :i do/. ( -n books for descriptions, in print, of what they 

 ought to have had visibly before them. Of course, when 

 a person becomes experienced, such aids are unnecessary, 

 but never useless. Our remarks have still more force 

 when applied to crystalline substances, in which the 

 external form varies so greatly. 



We shall now proceed to point out the various tests 



tSecnntr, p. 370. 



